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6.1C.1 Page 5 The monitoring offailures can be difficult during a storm. Response crews are focused on the safe and timely removal of fallen trees and debris. Some work is performed by contract crews called directly by dispatch so initial reporting may not be complete. The actual cause of each tree failure may not always be recorded fully or accurately. However, staff has tried to be diligent in monitoring the root pruned tree failures to learn about the amount of roots lost due to root pruning and if there is a correlation to failure. Staff attributes 67 tree failures to root pruning (1.4% of root pruned trees) over the history of the sidewalk repair program. The majority of these failures occurred during significant storms when other trees failed that were not root pruned. There are two tree species - Evergreen Chinese Elm (Ulmus parviflora) and Deodar Cedar (Cedrus deodara) - that have failed due to root pruning without storm events. Two other species also showed very shallow root systems - California Pepper (Schinus molle) and Tulip Tree (Liriodendron tulipifera) - that are regarded as highly prone to failure following root pruning. Staff's practice is to avoid root pruning these four species. The most significant tree failure event was in November 2003. During a severe storm 14 large whole tree failures occurred, of which nine trees had been root pruned, and five had not. Additionally, over 400 other calls for service for large broken limbs, tom branches, and private tree failures were recorded. During the recent December 31, 2005 storm, 63 service calls were received citywide for failed limbs and whole tree failures. Some of the 63 calls had multiple occurrences at an address. Eleven trees on private property failed, 8 large and 3 small. Four large City-owned trees failed. Two Ash trees located in the Woodside Plaza neighborhood had been root pruned, one approximately five years ago and the other approximately two years ago. The other two failures were Acacia trees located east of the railroad tracks, one along Chew Street and one along Blomquist. Fourteen small City-owned trees were destroyed at different locations throughout the City. No storm-induced failures occurred on trees that were root pruned during the current project. One Ash tree, root pruned during the current project, was observed to be leaning in late December 2005. Staff inspected, found the stability compromised, and removed the tree. One Ash tree located along Bristol Way, root pruned during the current project, failed during September 2005. After investigation, staff concluded the tree was hit by a debris box truck and also had over-saturated soil at the root crown. The root systems of the four root pruned trees (in the previous paragraph) were carefully excavated and the roots washed off. Staff found that the root pruning that occurred ranged from a distance of 36 inches from the trunk (target distance) to 28 inches from the trunk. The root pruning distances were directed by staff based on the available space present in order to perform the necessary sidewalk repairs. It was concluded that in these situations, the Contractor performed the root pruning as directed by the City. Decay was present in the base TreFFailure Data AnaluYSls Page 4 of 6 . ~. '-".- .--- --- .-. __._ .. _ .__..__~. .u_ _,_.,__ .-__~__'~.__._....M.______.____w_._,_.~_..__ _ ~~-_.---